Two high-power appliances on a single 40A RCD

A short pulse of high current has very little heating effect anywhere, and is basically the adiabatic formula at low current and very short period. You can test this yourself again by calculation.

My MFT on 25A loop test seems to heat things enough to reliably blow a 3A BS 1362 fuse. (As I discovered, the hard way :( )


  - Andy.

Alcomax

You appear to believe that domestic rings and radials shall not have more than 3 socket outlets of 13A or actually two.

David, you appear to believe your dreams are reality.

Alcomax

You appear to believe that domestic rings and radials shall not have more than 3 socket outlets of 13A or actually two. This is curious to say the least, and is NOT supported by BS7671. In the circuit under discussion there is no intention that Ib is > In, because the intention is that reasonable loading is applied, ie. either the shower or the cooker. This is the intention of BS7671, and you have failed in your reading of BS7671 in that both at once is not the normal use, which you yourself emphasise. The fact that it can happen is why we fuse for overload protection, using a suitable CPD.  You also need to be aware that MCBs have a specification which includes a mechanical life, and breaking something like their rated current or a little more causes no more damage than in any switch. Breaking their ultimate breaking capacity (say 6kA) may cause some damage but not enough to require that they need to be replaced after clearing the odd short circuit fault. As a design criterion it is not normally considered that overload of a circuit must be impossible, just reasonably unlikely in terms of cable heating, and protection is supplied by the CPD. There is a possible danger that an overload of up to 30% or so may not open the CPD, either at all or take a long time, and this is the long slow case which is often discussed. This level may get the cable somewhat hot, shortening life, but never hot enough to burn it, which you may calculate.


Sparkingchip

That idea (old wives tale) about fuses and circuit Zs testing has never been true, because it is contrary to the physics. A short pulse of high current has very little heating effect anywhere, and is basically the adiabatic formula at low current and very short period. You can test this yourself again by calculation. It is proved by table lamps correctly fused every day (Filament lamp of course).


Everyone

Here is something which is useful for every electrician to try, and is easy if you have an electronic temperature probe or thermometer. Take a piece of 1mm T&E cable and a 13A plug and socket strip (say 4 sockets) Find a number of high power appliances and run them via your 1mm "extension cable" for various periods while monitoring the current and temperature. You may be very surprised by your results, particularly if you plot them as multiple curves of time against temperature (similar to the MCB curves in the regs with temperature on the Y axis, time on the X axis and a curve for each current. You may post the results, they are quite interesting. (Loads to about 20A are suitable, you may need to bodge the plug fuse if heating the cable takes too long). Take care not to start a fire etc.

I always advise against having a 10.8 kW shower, generally there won’t be enough water.


Andy Betteridge

I think there's a big difference between e.g. an under-specced 16A kitchen circuit tripping every other day because whenever you turn on more than one or two of kettle, washing machine, toaster etc it exceeds the design current; and a shower+cooker circuit tripping once per year.


If we are to follow the literal letter of the "law", then we shouldn't install both a cooker and a shower in the same house which is protected by only a 60A cut-out fuse.  We should become po-faced and tell the customer that they must contact the DNO to get their supply upgraded to 80A plus before we install the shower.

Back in the days when all loop testing was carried out at high currents before there were RCDs in the circuits and installation some specifications said that fuses had to be replaced after the loop testing had been completed, because the stress put on them by the loop testing could lead to early failure.


So at one time some fuses such as 5 amp fuses in lighting circuits were considered scrap after having 20 amps whacked across them got a very short period during a loop test.


Circuit protective devices are not indestructible.


Andy Betteridge 

But normal service in this case is that cooker and shower won't be used together. It's no different than saying that in normal service, one won't put a large pan of cold water on every hob ring and simultaneously turn them and both ovens on. In either case, if the combined overload lasts too long then the cable won't reach an excessive temperature before the OCPD trips. After which, the unskilled householder in their uncontrolled house will deduce that you can't use all those things at the same time and avoid doing so again.